A gas turbine having in its simplest form a compressor, a compressor driving turbine, an output turbine and a fuel combustor is provided with a fuel supply circuit through which fuel is fed to the fuel combustor through a fuel injection valve projecting into the combustor. The flow rate of the fuel to be supplied from the fuel supply circuit to the fuel combustor of the gas turbine is continuously controlled by means of a fuel flow control valve provided in the fuel supply circuit. Throughout the operation of the gas turbine, the fuel flow control valve is controlled to continuously vary the flow rate of the fuel therethrough so as to enable the gas turbine to produce a constant output speed optimum for the amount of load applied to the gas turbine directly or through any equipment driven by the gas turbine. Thus, the gas turbine is further provided with an output control system for regulating the opening degree of the fuel flow control valve through detection of the output speed of the gas turbine. The detected output speed of the gas turbine is compared with a desired turbine output speed optimum for the amount of load being applied to the gas turbine and a signal is produced which is variable with the difference between the detected and desired output speeds of the gas turbine. Such a signal is fed to a proportional-plus-integral action circuit including a parallel combination of a proportional amplifier and an integrator having a predetermined time constant. The proportional-plus-integral action circuit is thus operative to produce an analog output signal which varies with the time integral of the difference between the detected and desired output speeds of the turbine. The signal delivered from the proportional-plus-integral action circuit is fed to the fuel flow control valve and control the valve in such a manner that the detected output speed of the gas turbine constantly converges toward the desired turbine output speed.
The integrator thus incorporated into an output control system of the above described nature is effective to eliminate the offset involved in the output signal delivered from the proportional amplifier connected n parallel with the integrator. The response of the integrator to an input signal can be improved by increasing the time constant of the integrator but, as the time constant of the integrator is increased, the quality of control by the output signal of the proportional-plus-integral action circuit deteriorates, as is well known in the art. The time constant of the integrator used in a proportional-plus-integral action circuit is, therefore, selected with a view to providing an acceptable compromise between the conflicting requirements for the quality of control and the quality of responsiveness.
Though the time constant of the integrator forming part of a proportional-plus-integral action circuit is selected to provide such a compromise, a problem is encountered in a proportional-plus-integral action circuit used in a control system for a gas turbine in that the fuel flow control valve provided in the fuel supply circuit for the gas turbine cannot promptly respond to an abrupt change in the amount of load on the gas turbine due to the delay involved in the response of the integrator to the variation in the signal applied to the integrator and the delay involved in the response of the fuel flow control valve to the signal applied to the valve. Due to the delayed response of the fuel flow control valve to abrupt variation in the amount of load on the gas turbine, the output speed of the gas turbine tends to fluctuate during a transient period of time subsequent to the abrupt change in the amount of load. If this takes place in a gas turbine used as a driving source for an electric power generator, the power generator is disabled to produce a current with a rated frequency for a certain period of time after the amount of load on the gas turbine has changed.
The delay involved in the response of the fuel flow control valve to the control signal applied thereto can be practically eliminated by provision of a fuel return valve arranged in series with the fuel flow control valve for reducing the flow rate of the fuel to be supplied to the combustor of the gas turbine in response to an abrupt decrease in the amount of load on the gas turbine or by means of a bypass valve arranged in shunt across the fuel flow control valve for permitting fuel to be fed to the combustor of the gas turbine by way of the bypass valve as well as the fuel flow control valve for some time after the amount of load on the gas turbine has changed. Provision of a fuel return or bypass valve in addition to the fuel flow control valve in a fuel supply circuit for a gas turbine is thus useful for maintaining the output speed of the gas turbine substantially unchanged for the transient period of time after the amount of load on the gas turbine is changed. When the fuel return or bypass valve closes at a certain time interval subsequent to the change in the amount of load on the gas turbine, however, the delay in the response of the integrator in the control circuit of the gas turbine comes into effect on the flow rate of the fuel to be supplied to the combustor of the gas turbine and in the result causes the output speed of the gas turbine to fluctuate.
An object of the present invention is to provide an improved turbine output control system in which the delay in the response of the integrator forming part of the control system is eliminated or at least reduced to a minimum by reducing the time constant of the integrator or superposing an additional input signal to the integrator during a transient period of time subsequent to the occurrence of an abrupt change in the amount of load on the gas turbine.